Method for stabilisation of metallic mercury using sulphur

ABSTRACT

Process for the stabilization of mercury metal by reaction of the mercury metal with sulphur in the solid state, in which the mercury and the sulphur are brought into contact, at an Hg/S molar ratio of 1/1 to 1/3, in a reactor integral with a hollow pipe in fluid communication with the interior space of the said reactor, the said hollow pipe comprising a first end connected to the wall of the said reactor and a second end distant from the said reactor; the said hollow pipe and the said reactor being hermetically sealed, the said hollow pipe being provided with rotating means external to the said pipe and to the said reactor for rotating the said reactor and the said pipe around the axis of the said pipe, and the said hollow pipe being provided, at its end distant from the reactor, with means for introducing the sulphur and the mercury inside the reactor and discharging the reaction products.

The invention relates to a process for the stabilization of mercurymetal with sulphur.

The process applies in particular to the stabilization of mercurycontaminated by radioelements.

The technical field of the invention may be defined as that of thetreatment of mercury-based waste for the purpose of its storage or ofits removal.

Direct storage or incineration is impossible because of the undesirableand toxic mercury vapours.

For this reason, processes targeted at reducing the mobility of mercuryin the environment have already been employed.

These immobilization processes are targeted essentially at preventingthe mercury from being released into the atmosphere, by volatilization,and into the soil, by leaching.

The immobilization processes are encapsulation, amalgamation andstabilization.

Encapsulation is the physical immobilization of the dangerous elementsby enveloping the waste in a non-porous and impermeable matrix toprevent the release of the mercury vapours and to also prevent theleaching of the mercury.

The known encapsulation processes are the use of cements based onsulphur polymers or phosphate ceramics. Polyethylene, polyesters,polysiloxane and others can also be used. These processes can include astage of chemical stabilization of the mercury.

Thus, the document U.S. Pat. No. 6,399,849 B1 from Brookhaven NationalLaboratory discloses the use of a cement based on sulphur polymers fortreating mercury. This process takes place in two stages:

-   -   the polysulphides of the sulphur polymer convert the mercury to        mercury sulphide (HgS);    -   the waste HgS formed is solidified in the cement.

Amalgamation is the physical immobilization of the mercury with anothermetal to form a semi-solid alloy or amalgam.

Mercury amalgamates with virtually all metals but more particularly withcopper, nickel, tin, zinc, gold or silver. The problem of this techniqueis that it does not significantly reduce the leaching and thevolatilization of the mercury. This process thus has to be followed byan encapsulation stage.

Stabilization is the chemical immobilization by combination withimmobile entities in order to reduce the release of the dangerouselements in the atmosphere or the biosphere. The process which forms thesubject-matter of the invention belongs more particularly to this typeof process for the treatment of mercury.

The commonest stabilization of mercury metal in the literature is thatwith sulphur, although this technique is commonly defined as anamalgamation technique in the patents of which it forms thesubject-matter. This term appears to be unsuitable because a reactionclearly takes place between the mercury and the sulphur, which has to bedescribed as “stabilization”.

A highly water-insoluble (0.0125 mg/l) mercury sulphide or metacinnabaris thus formed, which is converted to red sulphide or cinnabar byheating (at a temperature of 386° C.).

In the case of mercury in the +II oxidation state, stabilization isobtained by precipitation with sodium sulphide or hydrogen sulphide orwith trimercapto-s-triazine.

The disadvantage of these techniques is the need for preoxidation of themercury to the +II oxidation state in aqueous solution.

It is thus preferable to use, when it is desired to treat elementalmercury metal, as is the case in the present document, the technique ofstabilization between elemental mercury and solid sulphur according tothe following reaction:Hg+S→HgS

The document U.S. Pat. No. 5,034,054 discloses a process in whichmercury metal is mixed with an inorganic powder which results inpermanent bonding of the mercury to the powder in the solid form. The“amalgam” obtained can be easily and without risk stored in a landfillsite.

The inorganic powder is chosen from powders formed of transition metalswhich are reactive with mercury, and sulphur powder.

The sulphur powder is added to the mercury in an S:Hg ratio of at least1:1 and preferably of the order of at least 3:1, by weight.

The inorganic powder and the mercury are preferably placed in adisposable container and are subjected to nonintrusive stirring.

The container with the amalgam is subsequently stored in a landfillsite.

The nonintrusive stirring is carried out by placing the disposablecontainer in a device of the paint mixer type which produces stirringalong three axes.

It is mentioned that other types of nonintrusive stirring might possiblygive good results, without further details.

The devices used in this document are not hermetic.

The document by R. Stewart et al., “Proceedings of the InternationalConference on Decommissioning and Decontamination and on Nuclear andHazardous Waste Management”, Denver, 13-18 Sep. 1998, volume 3, pages33-36, reports the results obtained during tests “on amalgamation” ofmercury with- sulphur. Liquid mercury is stabilized with sulphur in theform of powder in a commercial mill mixer. The addition of mercury takesplace in a controlled way onto the sulphur.

The temperature of the mixture is measured continuously and samples areanalysed periodically in order to determine the amount of free mercury.The reaction is carried out at ambient temperature.

The processes described in the two abovementioned documents involve thefollowing reaction:Hg+S→HgS.

In the processes of these two documents, the sulphur is added in a largeexcess with respect to the mercury, namely with an Hg/S ratio of 1/1 to1/3 by weight (which corresponds to an Hg/S molar ratio of 1/6.5 to1/19). The consequence of this large excess of sulphur is the formationof a very large volume of HgS and sulphur product.

In addition, the processes of these two documents do not guarantee theabsence of mercury vapours in the area where the reaction is beingcarried out and specific protection of personnel and/or ventilation aretherefore necessary in carrying out these processes.

The document by Lawrence N. Oji, “Mercury Disposal via SulfurReactions”, Journal of Environmental Engineering, October 1998, pages945-952, relates to the treatment of mercury, in particular ofradioactive mercury, with sulphur for the purpose of the storagethereof.

Elemental mercury is converted to its sulphides by mixing the mercurywith sulphur powder in a high shear mixer with stirrer blade speeds ofup to 19 000 revolutions/min or in a reactor with blade speeds of atleast 1000 revolutions/min.

HgS is thus prepared in the α and β crystalline forms (cinnabar andmetacinnabar); the reactor makes possible only the production of blackmercuric sulphide (metacinnabar), whereas the high shear mixer makes itpossible to prepare equally well black mercuric sulphide as red mercuricsulphide.

It is necessary, with the high shear mixer, to continuously maintaincooling and inert atmosphere conditions to prevent the danger ofcombustion of the sulphur and of fire from increasing. With the reactor,it is only possible to obtain black mercuric sulphide because of thelimited rotational speed of the blades, the fixed volume of air and thecirculation of cooling water in the reactor.

The conditions for carrying out the process described in the document byOji are extreme conditions; this is because it requires systems forrendering inert, for cooling with water and for measuring thetemperature. It also requires a safety valve and continuous monitoringof the rise in temperature and in pressure of the system. In addition tothe conditions for carrying out the process, another disadvantage of thelatter is the impossibility of extracting tritium oxide or tritiatedwater.

It emerges from the above that there exists a need for a process for thestabilization of mercury metal by reaction of mercury metal with sulphurin the solid state which is simple, easy to carry out, reliable, safe,of short duration and inexpensive and which provides a high reactionyield.

There exists in particular a need for such a process which provideshighly effective protection of the personnel carrying out the saidprocess without having recourse to complex devices.

There also exists a need for a process which generates a reduced volumeof final mercury sulphide HgS product in order to reduce the handling,intermediate storage and storage costs of such a product.

This final product should exhibit the lowest possible sensitivity toleaching tests and should improve the results obtained in this fieldwith the processes of the prior art.

In the specific case of radioactive mercury, the process should make itpossible to reduce the contamination of the mercury sulphide formedafter stabilization.

The aim of the present invention is to provide a process for thestabilization of mercury metal by reaction of the mercury metal withsulphur in the solid state which meets, inter alia, the needs listedabove and which satisfies the requirements and criteria mentioned above.

This aim and yet others are achieved, in accordance with the presentinvention, by a process for the stabilization of mercury metal byreaction of the mercury metal with sulphur in the solid state, in whichthe mercury and the sulphur are brought into contact, at an Hg/S molarratio of 1/1 to 1/3, in a reactor integral with a hollow pipe in fluidcommunication with the interior space of the said reactor, the saidhollow pipe comprising a first end connected to the wall of the saidreactor and a second end distant from the said reactor; the said hollowpipe and the said reactor being hermetically sealed, the said hollowpipe being provided with rotating means external to the said pipe and tothe said reactor for rotating the said reactor and the said pipe aroundthe axis of the said pipe, and the said hollow pipe being provided, atits end distant from the reactor, with means for introducing the sulphurand the mercury inside the reactor and discharging the reactionproducts.

The process according to the invention is defined by a combination ofspecific characteristics.

It should first of all be noted that the process according to theinvention employs mercury metal and sulphur in the solid state and notan aqueous solution of Hg(II) which requires, for its preparation, apreliminary oxidation of the mercury.

The process according to the invention generates a smaller volume ofproduct than in the prior art.

The process according to the invention employs a specific Hg/S molarratio of 1/1 to 1/3, which corresponds to a weight ratio of 1/0.16 to1/0.48. Such a molar ratio makes it possible to reduce the volume ofproduct formed by a factor of between 4 and 12, depending on thestoichiometry, with respect to the processes of the prior art, such asthat disclosed in the document U.S. Pat. No. 5,034,054.

The process according to the invention is carried out in a specificreactor which is firmly attached to (integral with) a hollow pipe influid communication with the interior space of the said reactor.

The said hollow pipe is provided with rotating means external to thesaid pipe and to the said reactor for rotating the said reactor and thesaid pipe around the axis of the said pipe.

This reactor, which incorporates a pipe acting as a hollow stirring pipeto which it is firmly attached (integral), is very simple.

Stirring is of the nonintrusive type. It is rotational stirringinvolving rotating means, such as a motor, which are external to thepipe and to the reactor and which are completely independent of these.These means, unlike intrusive stirring systems, are thus notcontaminated by the reactants and do not have to be changed between eachtreatment batch, thus increasing the volume of the waste generated.

No heating is brought about by the rotational stirring of the invention,in contrast to the stirring system used in the document by Oji, whichproduces a great deal of heat and which requires rendering inert and thepresence of a cooling system.

In addition, it should be noted that the stirring employed according tothe invention does not require complex means for rendering leaktight atthe point where the stirrer passes through the wall of the reactor asthe stirring is generated by means entirely external to the reactor.

The rotational stirring according to the invention is much easier tocarry out than the stirring according to three axes disclosed in thedocument U.S. Pat. No. 5,034,054 and is more efficient.

Due to the specific structure of the reactor employed in the process ofthe invention and the external rotational means with which it isprovided, the reactor can easily be equipped with means for introducingor charging the reactants, namely the sulphur and the mercury, insidethe reactor and removing or discharging the product resulting from thereaction, namely HgS, as complete reaction occurs between the sulphurand the mercury.

These means are provided at the end of the hollow pipe distant from thereactor. It is not necessary to disconnect the stirring system and thereactor in order to introduce or remove the reactants and products fromthe latter as the charging/discharging is carried out via the hollowstirring pipe or shaft which is firmly attached to the reactor and whichis formed as one piece with the latter. The charging/discharging can bemanual or automatic and is greatly simplified in comparison with theprocesses of the prior art.

The reactor and the pipe are hermetically sealed, which ensures, incontrast to the devices of the prior art, completely safe operation ofthe process, both for the environment and for the personnel.

The hermetically sealed reactor and pipe make it possible to isolate thepersonnel from the mercury vapours and to prevent recourse having to behad to a ventilation system.

Advantageously, the reactor is an essentially spherical reactor. Such areactor is generally described as a “round-bottomed” reactor (“ball”).

Advantageously, the reactor is equipped with heating means and/or withcooling means. Heating makes it possible to accelerate the reaction ofthe mercury with the sulphur.

The reaction is generally carried out at a temperature of 20 to 100° C.,preferably of 60 to 80° C., at atmospheric pressure.

The main axis of the pipe can be coincident with the axis of thereactor. Stirring is thus produced by symmetrical rotation around thisaxis.

The main axis of the pipe can also be offset with respect to the axis ofthe reactor.

The process according to the invention applies to the treatment ofmercury metal.

The mercury employed in the process according to the invention can bemercury contaminated by volatile and/or nonvolatile, for example solid,impurities or contaminants.

It should be noted that large amounts of mercury metal and of solidmercury waste are present on nuclear sites, for example in France.

The process according to the invention applies very particularly to thetreatment of mercury contaminated by radioelements, the nature and thecontent of which in the mercury can vary. They may be volatileradioelements, for example tritium in the form of tritiated water, orheavy nonvolatile radioelements, for example plutonium and/or uranium inthe form of oxides.

The process according to the invention can also be used formercury-comprising solid waste.

Prior to the reaction with the sulphur, the mercury (alone) can bedistilled in the case where the latter is contaminated or is present insolid waste.

This distillation proves to be particularly useful in the case where themercury is mercury contaminated by radioelements as it makes it possibleto reduce the radiological contamination of the mercury sulphide (HgS)formed after stabilization.

In the case where the mercury is contaminated by volatile contaminants,this distillation, which is carried out on the mercury alone before anycontact with the sulphur, can be carried out at a moderate temperature,namely at a temperature generally from 90 to 120° C., at atmosphericpressure.

In this case, it is thus essentially the volatile contaminants presentin the mercury, such as water, for example tritiated water, which areremoved.

This distillation is preferably carried out in the same reactor wherethe reaction takes place before introduction of the sulphur into thesaid reactor to form HgS.

In order to carry out this distillation at moderate temperature,distillation means are provided on the said hollow pipe.

In the case where the mercury is contaminated by “heavy” nonvolatilecontaminants or is present in solid waste, this distillation can becarried out at a high temperature, namely, for example, of 360° C., atatmospheric pressure, by means of which it is then the mercury which isdistilled and which is collected in a recovery receiver (round-bottomedreceiver). The nonvolatile impurities or contaminants, such as theoxides of the heavy elements Pu and U, remain in the reactor where thedistillation has taken place and are recovered. The mercury is thusisolated from the “clean” solid waste.

The reaction between the mercury and the sulphur can then be carriedout, in accordance with the process according to the invention, in thereceiver, such as a round-bottomed receiver, where the distilled mercuryhas been collected and where it is present, the said receiver thenacting as the reactor used in the process.

In the case where the volatile contaminants or the mercury aredistilled, the distillation temperatures can be greatly lowered byreducing the pressure inside the receiver where the distillation takesplace, for example the reactor, to a pressure below atmosphericpressure, for example by addition of a vacuum pump.

The reactor used according to the invention, which is hermeticallysealed, actually makes it possible to operate in that way and to carryout a distillation under reduced pressure (with respect to atmosphericpressure).

The invention will be better understood on reading the description whichwill follow, given by way of illustration and without impliedlimitation, with reference to the appended drawing, in which:

FIG. 1 is a diagrammatic view in partial cross section of a reactor forthe implementation of the process according to the invention.

In FIG. 1, a reactor as used to carry out the process of the inventionhas been represented. The reactor (1) as such comprises a chamber,preferably an essentially spherical chamber, the wall (2) of which ismade of a material which is inert with respect to the reactants, namelywith respect to sulphur and mercury metal, for example glass.

This generally spherical chamber is firmly attached to a hollow pipe (3)with a generally circular cross section. The interior space (4) of thereactor (1) and the interior space (5) of the pipe are in fluidcommunication via an orifice (6), generally a circular orifice, made inthe wall (2) of the reactor. The said pipe (3) comprises a first end (7)connected to the wall (2) of the reactor (1) where the walls (8) of thesaid pipe and the wall (2) of the reactor (1) meet and a second end (9)distant from the reactor.

The wall (2) of the reactor and the walls (8) of the pipe can be formedas one piece; alternatively, the connection can be made via-aconventional ground joint.

Means for introducing the reactants into the reactor and/or removing thereaction products from the reactor are provided at the end (9) of thehollow pipe distant from the reactor.

These means generally comprise a sealing valve (10) which makes itpossible to hermetically seal the combined reactor (1) and pipe (3).

The reactor and the pipe are sealed (airtight). In other words, all thecomponents in contact with the mercury or liable to be in contact withthe latter are confined.

The reactor is equipped with rotating means which are, according to theinvention, means external to the said pipe and to the said reactor, thatis to say means independent of the reactor part.

These means also comprise a motor (11) and a system of gears (notvisible in the drawing) intended to impart rotational movement to thecombined reactor and pipe.

The device according to the invention can also comprise distillationmeans. These distillation means generally comprise a condenser (verticalcolumn) which is attached, for example by screwing, to the hollow pipe.This column is thus different from the hollow pipe firmly attached tothe reactor.

Alternatively, it is possible to provide a recovery round-bottomedreceiver for recovering the mercury with a cold trap, in the case wherea preliminary distillation of the mercury is carried out at a hightemperature, for example at 360° C.

The recovered mercury, with the sulphur, then reacts in the saidrecovery round-bottomed receiver in accordance with the process of theinvention or else the mercury is transferred into another reactor, forexample a round-bottomed reactor, in which it reacts with the sulphur inaccordance with the process of the invention.

The said condensers or recovery round-bottomed receivers are generallyconnected to the hollow pipe via an additional connection (12) equipped,for example, with a threaded joint, as indicated above.

The reactor can also comprise additional heating means (notrepresented), for example in the form of an electrical systemsurrounding the reactor.

The reactor can also comprise cooling means, for example in the form ofa bath of ice-cold water or of a container filled with ice surroundingthe reactor.

In the case of treatment of mercury with sulphur, these cooling meansare not used.

Means for placing under vacuum can be added via the additionalconnection (12) generally present at the end of the hollow pipe mostdistant from the reactor.

In FIG. 1, the device comprising the reactor, the hollow pipe, the motorand the gearings, and the like, is attached to a fixing rod (13) butother fixing means can obviously be used.

The invention will now be described with reference to the followingexamples, given by way of illustration and without implied limitation:

EXAMPLES

In the examples which follow, the process according to the invention iscarried out with nonintrusive stirring. The arrangement used correspondssubstantially to that described in FIG. 1, with the reactor having theform of a round-bottomed reactor with a volume of 50 ml, 100 ml, 250 mlor 2000 ml and heating means composed of electrical heating systemssurrounding the round-bottomed reactor.

Examples 1 to 5 are carried out on the laboratory scale (with an initialweight of mercury of approximately 55 g). In Example 7, the reaction iscarried out on the semi-pilot scale with a weight of mercury ofapproximately 1 kg. In Example 6, the influence of the stirring speed isstudied on the laboratory scale and on the semi-pilot scale.

In Example 8, the analyses of the finished product and of the eluateresulting from the leaching of the finished product are reported.

In Example 9, the results of the analyses described in Example 8 arecompared with the regulatory criteria for acceptance into storage.

Example 1

In this example, the influence of the S/Hg stoichiometry on the reactionis studied.

A first experiment consisted in reacting sulphur and mercury in an S/Hgmolar ratio of 1.5 in a 100 ml reactor at a rotational speed of 50revolutions/min at ambient temperature.

After stirring for 24 h, a gaseous mercury measurement indicates thatthe reaction is not complete. This is because the Hg(g) content isgreater than 2000 μg/m³. This experiment is thus halted.

A second experiment was carried out with an S/Hg molar ratio of 3 in a100 ml reactor at a rotational speed of 50 revolutions/min at ambienttemperature. The same observation is made as above after stirring for 24h.

These two experiments show that the rate of reaction between the mercuryand sulphur is slow at ambient temperature. The timescale forobservation of the experiments is not sufficient to show any influenceof the stoichiometry on the rate of the reaction at ambient temperature.

Example 2

In this example, the influence of the temperature of the reaction mediumon the reaction is studied.

A series of experiments was carried out with an S/Hg molar ratio of 1 ina 50 ml reactor with a rotational speed of 50 revolutions/min and attemperatures of 40° C., 60° C. and 80° C.

After stirring for 24 h, a gaseous mercury measurement for each of theexperiments is carried out. The results are as follows:T=40° C.: Hg(g) content>2000 μg/m³T=60° C.: Hg(g) content=1500 μg/m³T=80° C.: Hg(g) content=70 μg/m³

This series of experiments shows that the rate of the reaction betweenthe mercury and the sulphur increases with the temperature.

Example 3

In this example, the influence of the free volume in the reactor on thereaction is studied.

A series of experiments made it possible to demonstrate that, for thesame rotational speed of the reactor, the greater the diameter of thereactor, the more the mercury and the sulphur are plastered onto thewalls, slowing down the kinetics of the reaction. This is because thecentrifugal force increases with the diameter of the reactor.

Example 4

In this example, the influence of an indentation in the reactor on thereaction is studied.

Several experiments using a reactor possessing indentations, withdifferent S/Hg molar ratios, made it possible to demonstrate an increasein the rate of the reaction between the sulphur and the mercury. This isbecause the indentations on the reactor fragment the mercury dropinitially present in the reactor into small beads. This fragmentationfacilitates the contact and the reaction between the mercury and thesulphur.

Example 5

In this example, the influence of the presence of grinding agents on thereaction is studied.

Experiments were carried out with an S/Hg molar ratio of 1 in a 50 mlreactor.

The grinding agents tested are Fontainebleau sand (9 g) and glass beadswith a diameter of 5 mm. The introduction of grinding agents has theeffect of compacting the sulphur and the mercury instead of fragmentingthe reactants. This formation of aggregates greatly slows down the rateof reaction.

Example 6

In this example, the influence of the stirring rate of the reactor onthe reaction is studied.

Several series of experiments on the laboratory scale (and on thesemi-pilot scale) made it possible to demonstrate the influence of thestirring rate of the reactor on the kinetics of the reaction.

For example, on the laboratory scale, experiments were carried out withan S/Hg molar ratio of 3, with a weight of mercury of approximately 55g, in a 100 ml reactor, at a temperature of 80° C., for stirring ratesof the reactor of 200 revolutions/min and 60 revolutions/min.

After stirring for 24 h, a gaseous mercury measurement for each of theexperiments is carried out. The results are as follows:s=200 revolutions/min: Hg(g) content=550 μg/m³s=60 revolutions/min: Hg(g) content=120 μg/m³These experiments thus show that, when the rotational speed of thereactor is reduced, the kinetics of the reaction are faster. In the casewhere the rotational speed of the reactor is low, the centrifugal forceis not sufficient to plaster the reactants against the walls of thereactor. Thus, the powder composed of the reactants falls back down inthe round-bottomed reactor at each rotation of the reactor. The mixingbetween the mercury and the sulphur is thus very efficient.

Example 7

In this example, the reaction is carried out on the semi-pilot scale.

In the preceding examples (1 to 5 and, in part, 6), examples on thelaboratory scale with a weight of mercury in the vicinity of 55 g weredescribed.

In the example described below, the reactions are carried out on thesemi-pilot scale, that is to say that the initial amount of mercury isof the order of 1 kg.

The object of the semi-pilot trials is to confirm, on the semi-pilotscale, the parameters determined on the laboratory scale. Twoexperiments were carried out:

Experiment 1

S/Hg=1.2 (initial weight of mercury=1 kg)

Volume of the reactor=2 l

Temperature=60° C.

Reactor with smooth walls

Rotational speed of the reactor=200 revolutions/min

Experiment 2

S/Hg=1.2 (initial weight of mercury=1 kg)

Volume of the reactor=2 l

Temperature=60° C.

Reactor with indentations

Rotational speed of the reactor=50 revolutions/min

In the case of Experiment 1, after stirring for 5 h, the gaseous mercurycontent is 200 μg/m³. The finished product exists, in equivalentproportions, in the form of large (a few centimetres) dark-greyaggregates and of a fine powder dark-grey in colour. A few beads ofmercury are still visible in the aggregates.

In the case of Experiment 2, after stirring for 2 h, the gaseous mercurycontent is 200 μg/m³. The finished product exists in the form of a veryfinely divided black powder.

These two experiments make it possible to confirm the parameters of thereaction. The analyses on the finished products resulting from these twoexperiments (cf. Examples 8 and 9) are compatible with the criteria foracceptance set by legislation.

However, the addition of indentations to the reactor and the use of arotational speed of the reactor of the order of 50 revolutions/minincrease the kinetics of the reaction and reduce the particle size ofthe finished product. These experiments on the semi-pilot scale havethus made it possible to confirm the parameters for reaction between thesulphur and the mercury. The process is thus applicable on theindustrial scale to amounts of the order of several kg of mercury.

Example 8

In this example, analysis of the finished product and of the eluateresulting from the leaching of the finished product is carried out.

The analyses carried out on the finished product are an X-raydiffraction analysis and a measurement of the dryness.

The finished product was subsequently subjected to a leaching test. Theanalyses carried out on the eluate, that is to say on the leachingsolution, are a measurement of the pH and an analysis of the mercurycontent.

Analysis by X-ray Diffraction

A powder X-ray diffraction diagram for the samples formed of powderobtained in accordance with the process of the invention was produced.The Hg/S stoichiometry is 1.5. The peaks characteristic of metacinnabarHgS and of sulphur (added in excess) are present in the diagram. Thisanalysis confirms that the reaction is complete and that the productformed is mercury sulphide.

Measurement of Dryness

Dryness measurements were carried out according to standard NF ISO 11465on various samples of finished product obtained in accordance with theprocess of the invention, for Hg/S stoichiometries of 1.2. On average,the content by weight on a dry basis is 99.93% and the content by weightof water is 0.07%.

Leaching Test

Leaching tests were carried out according to standard NF EN 12457-2 onthe finished products from Experiments 1 and 2 of Example 7 obtained inaccordance with the process of the invention on the semi-pilot scale.

In the case of Experiment 1, the particle size of the finished productwas reduced in accordance with the standard applied (95% by weight ofthe particles must have a size of less than 4 mm). The finished productresulting from Experiment 2 (very finely divided powder) was notsubjected to particle size reduction.

After stirring for 24 h and separating by settling, the eluate isfiltered through a 0.45 μm membrane filter according to the standardalready mentioned.

The eluates resulting from the teachings of the finished productsresulting from Experiments 1 and 2 are subsequently analysed separately.

Measurement of the pH of the Eluate

The pH measurements are carried out on the eluates resulting from theTeachings of the finished products from Experiments 1 and 2 according tostandard NFT 90008. The results are as follows:

-   -   Eluate Experiment 1: pH=4.55    -   Eluate Experiment 2: pH=4.25

The acidity of the eluates can be explained by the presence of a slightexcess of sulphur in the finished product (Hg/S=1.2).

Mercury Content of the Eluate

The measurements of mercury content in the eluates resulting from theTeachings of the finished products from Experiments 1 and 2 of Example 7are carried out according to standard NF EN 1483. The ultrasounddigestion method is used. After reducing with hydroxylaminehydrochloride and stannous chloride, the mercury is entrained by a gasstream at ambient temperature (cold vapours). This procedure isdescribed in the abovementioned standard. The mercury content issubsequently calculated after measuring the absorbance of the mercury ata wavelength of 253.4 nm. The mercury contents in the eluates are asfollows:

-   -   Eluate Experiment 1: mercury content of the eluate=0.74 mg/kg of        finished product    -   Eluate Experiment 2: mercury content of the eluate=0.46 mg/kg of        finished product

The mercury contents of the eluates are low, which confirms, that themercury is well stabilized by the sulphur in agreement with thesolubility product of mercury sulphide HgS of 10⁻⁵⁴ mentioned in theliterature. The presence of a few beads of mercury in the finishedproduct from Experiment 1 does not to a disadvantageous extent increasethe mercury content of the eluate with respect to Experiment 2. It wouldappear that, even if these few drops of mercury are observed in thefinished product from Experiment 1, the mercury is sufficientlyimmobilized in the matrix for the effect of the leaching to benegligible.

Example 9

In this example, a study is carried out on whether the mercury sulphideobtained by the process according to the invention satisfies theregulatory criteria for acceptance into storage.

Mercury sulphide is a dangerous waste within the meaning of Decree No.2002-540 of 18 Apr. 2002 relating to the classification of wastespecifying how Article L.541-24 of the Law on the Environment should beenforced.

It can be accepted into storage if it observes the following thresholds:

-   -   4<pH<13: measurement carried out on the eluate Dryness>30%    -   Mercury content: Hg<2 mg/kg of dry stabilized waste.

The analyses carried out according to the standardized methods (cf.Example 8) show that the thresholds for acceptance into storage areobserved. The mercury stabilized by the process of the invention is thusdirectly accepted on a storage site.

1. A process for the stabilization of mercury metal by reacting themercury metal with sulphur in the solid state, in which the mercury andthe sulphur are brought into contact, at an Hg/S molar ratio of 1/1 to1/3, in a reactor integral with a hollow pipe in fluid communicationwith the interior space of said reactor, said hollow pipe comprising afirst end connected to the wall of said reactor and a second end distantfrom said reactor; said hollow pipe and said reactor being hermeticallysealed, said hollow pipe being provided with rotating means external tosaid pipe and to said reactor for rotating said reactor and said pipearound the axis of said pipe, and said hollow pipe being provided, atits end distant from the reactor, with means for introducing the sulphurand the mercury inside the reactor and discharging the reactionproducts.
 2. The process according to claim 1, wherein the reactor is anessentially spherical reactor.
 3. The process according to claim 1,wherein the reactor is additionally equipped with heating means and/orwith cooling means.
 4. The process according to claim 1, wherein thereaction is carried out at a temperature of 20 to 100° C. at atmosphericpressure.
 5. The process according to claim 1, wherein the main axis ofsaid pipe is coincident with the axis of said reactor.
 6. The processaccording to claim 1, wherein the main axis of said pipe is offset withrespect to the axis of said reactor.
 7. The process according to claim1, wherein the mercury is mercury contaminated by volatile and/ornonvolatile contaminants.
 8. The process according to claim 1, whereinthe mercury is mercury contaminated by radioelements.
 9. The processaccording to claim 1, wherein the mercury is present in solid waste. 10.The process according to claim 7, wherein, prior to the reaction withthe sulphur, the mercury alone is distilled.
 11. The process accordingto claim 10, wherein the mercury is contaminated by volatilecontaminants and the distillation is carried out at a moderatetemperature, at atmospheric pressure to remove essentially the volatilecontaminants present in the mercury.
 12. The process according to claim11, wherein the distillation is carried out in the reactor beforeintroduction of the sulphur into the reactor.
 13. The process accordingto claim 12, wherein, in order to carry out the distillation,distillation means are provided on said hollow pipe.
 14. The processaccording to claim 10, wherein the mercury is contaminated bynonvolatile contaminants and the distillation is carried out at a hightemperature, at atmospheric pressure by means of which the mercury isdistilled and is collected in a receiver, and the nonvolatilecontaminants are recovered, wherein, either: 1) the receiver is thereactor; or 2) the receiver is not the reactor, and the mercury istransferred from the receiver to the reactor for reacting with thesulphur.
 15. The process according to claim 14, wherein the receiver isthe reactor, and wherein the reaction of the mercury with the sulphur iscarried out in the receiver where the distilled mercury has beencollected.
 16. The process according to claim 10, wherein thedistillation is carried out at a pressure below atmospheric pressure.17. The process according to claim 4, wherein the reaction is carriedout at a temperature of 60 to 80° C.
 18. The process according to claim11, wherein the distillation is carried out at a temperature of 90 to120° C.
 19. The process according to claim 14, wherein the distillationis carried out at 360° C.
 20. The process according to claim 14, whereinthe receiver is a round-bottomed receiver.